It is desirable that fuel tanks have a residual volume of vapor or air trapped above the fuel in the tank after the tank has been completely filled. The space occupied by a trapped volume of vapor is called the ullage space. One way of achieving an ullage space is for the fuel filler pipe to enter the tank from a side surface rather than the top surface. When the level of fuel in the tank rises above the fuel filler pipe opening, the tank cannot be filled further unless the air or vapor trapped above the fuel filler pipe entrance is allowed to be displaced. Because of constraints on fuel tank shape and placement, it is not always possible to design the fuel filler pipe entrance to coincide with the desired ullage space and it is desirable to fill the tank above the fuel filler pipe entrance. This can be achieved by providing an escape passage which emerges from the top of the tank, which passage is itself closed off by the rising fuel level at a point designed to provide a limit to the level to which fuel can rise in the tank. This passage is referred to as a sensing tube.
Conventionally, the sensing tube comprises three parts, namely, a stub tube projecting into the fuel tank, a stub tube connected into the filler pipe, and a flexible hose connecting the two. Ideally, the open end of the sensing tube, inside the fuel tank, is positioned at a point where, to a substantial extent, the maximum level of fuel is unaffected by the attitude of the tank. In the past, this has led to the sensing tube passing through the top of the tank at its center and the opening of the tube has been located centrally at a given level in the fuel tank so that the fuel level is unaffected by the attitude of the vehicle carrying the tank.
A separately manufactured sensing tube exterior of the tank creates manufacturing as well as packaging difficulties. Top mounted sensing tubes either require greater clearance for the fuel tank or hollow areas on the tank surface to recess the tube. These recesses reduce the overall tank volume. This invention provides a simple way of avoiding entry of the sensing tube into the top of the fuel tank.